1. Field of the Invention
This invention relates to a radiation image read-out and reproducing system for scanning a recording medium such as a stimulable phosphor sheet, on which a single radiation image or a plurality of radiation images are recorded, with a light beam, obtaining image signals which carry information about the radiation images from the scanning process, and reproducing visible images by use of the image signals. This invention particularly relates to a method for recognizing the layout pattern of radiation images recorded on the recording medium. This invention also relates to a method for automatically recognizing a subdivision pattern of radiation images recorded on the recording medium such as a stimulable phosphor sheet.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image and obtaining an image signal therefrom, carrying out appropriate image processing of the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Unexamined Patent Publication No. 54(1979)-121043, an X-ray image is recorded on an X-ray film having a gamma value designed so as to match image processing, the X-ray image is read out from the X-ray film and converted into an electric signal, the electric signal (image signal) is image-processed and then used for reproducing the X-ray image as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality such as high contrast, high sharpness and high graininess can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored when it was exposed to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in proportion to the amount of energy stored during exposure to radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal, and by using the image signal the radiation image of the object is reproduced as a visible image on a recording material such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
The radiation image recording and reproducing system using a stimulable phosphor sheet is advantageous over conventional radiography using a silver halide photographic material in that an image can be recorded for a very wide range (latitude) of radiation energy intensities. More specifically, since the amount of light emitted upon stimulation after the radiation energy is stored on the stimulable phosphor varies over a wide range and is proportional to the amount of said stored energy, it is possible to obtain an image having a desirable density regardless of the energy intensity range of the radiation to which the stimulable phosphor sheet was exposed, by reading out the emitted light with an appropriate read-out gain and converting it into an electric signal to reproduce a visible image on a recording medium or a display device.
In order to detect the image signal correctly, the energy intensity range of the exposing radiation and other conditions must be determined from the recorded radiation image. Therefore, the aforesaid radiation image recording and reproducing system may be constituted such that a preliminary read out for approximately ascertaining the radiation image stored on the stimulable phosphor sheet is carried out by scanning the stimulable phosphor sheet with a light beam of a comparatively low energy level, and a preliminary read-out image signal, obtained from the preliminary read-out process, is analyzed. Thereafter, the final read out for obtaining the image signal used in reproducing a visible image is carried out by scanning the stimulable phosphor sheet with a light beam of a comparatively high energy level and reading out the radiation image under conditions adjusted on the basis of the results of an analysis of the preliminary read-out image signal.
Regardless of whether the preliminary read out is or is not carried out, it has also been proposed to analyze the preliminary read-out image signal or the image signal obtained and thereafter to adjust the conditions under which processing of the image signal is carried out on the basis of the results of the analysis of the image signal. The proposed method is applicable to cases where an image signal is obtained from a radiation image recorded on a recording medium such as conventional X-ray film, as well as to systems using stimulable phosphor sheets.
In the aforesaid radiation image recording and reproducing system, a plurality of radiation images are often recorded on a single recording medium such as a stimulable phosphor sheet for the purpose of efficiently utilizing the recording medium or for comparing radiation images of a single object recorded at different angles.
If it is not known that a plurality of radiation images exist on a single recording medium or the layout of the images on the medium is not known before the radiation images are read out from a single recording medium or are image-processed, the conditions under which a final read out is conducted and/or the conditions under which an image signal is processed are adjusted such that all of the radiation images recorded on a single recording medium are regarded as a single radiation image. In these cases, few problems arise when the image recording conditions and the consequent required image processing do not differ very much among the radiation images recorded on a single recording medium. However, when the image recording conditions and the consequent required image processing differ greatly among the radiation images recorded on a single recording medium, adjusting the conditions under which a final read out is conducted and/or the conditions under which an image signal is processed, as if all of the radiation images recorded on a single recording medium were a single radiation image, is unsuitable. As a result, visible images are reproduced with unsuitable densities, or images unsuitable for viewing purposes are reproduced because of inappropriate processing of the image signals.
These problems may be eliminated by inputting the number and layout of the radiation images recorded on the recording medium from a keyboard or the like into an apparatus before the radiation images are read out from the recording medium. However, the inputting operation is troublesome. Also, when an operator fails to input or erroneously inputs the number and layout of the radiation images recorded on the recording medium, the same problems described above arise, and the operator is often required to carry out the recording of an image again.
Also, in the aforesaid radiation image recording and reproducing system, in order to eliminate various influences caused by variations in the image recording conditions and/or to obtain a radiation image having a good image quality with enough accuracy to aid in efficient diagnoses of illnesses, it is desirable to ascertain the conditions under which the radiation image was stored on the stimulable phosphor sheet such as, for example, the radiation energy intensity levels used for image recording, or the image input pattern which is determined by the portion of the object (e.g. the chest or the abdomen of a human body) or the image recording method used, such as plain image recording or contrasted image recording, before reproducing the radiation image as a visible image, and then to adjust the read-out gain to an appropriate value based on the ascertained image recording conditions or the image input pattern. The image recording conditions and the image input pattern will hereinafter be referred to simply as the image input information when they are referred to generically. It is also desirable to adjust the scale factor to optimize the resolution in accordance with the contrast in the image input pattern.
The ascertaining of the image input information may be carried out prior to the reproduction of the visible image by use of the method disclosed in Japanese Unexamined Patent Publication No. 58(1983)-67240. In this method, a preliminary read out for ascertaining the image input information of a radiation image stored on a stimulable phosphor sheet is carried out in advance by use of stimulating rays having a stimulation energy of a level lower than the level of the stimulation energy of the stimulating rays used in the final read out from which a visible image for viewing is obtained, which visible image is used particularly for diagnostic purposes. After the preliminary read out the final read-out is carried out. In the final read out, the read-out gain and/or the scale factor is adjusted to an appropriate value, and/or some type of appropriate signal processing is conducted, on the basis of the image input information obtained in the preliminary read out.
Various methods may be used for approximately ascertaining the image input information from the preliminary read-out image signal obtained by performing the aforesaid preliminary read out. One method is to utilize a histogram of the preliminary read-out image signal. Specifically, the image input information can be ascertained from, for example, a characteristic value such as the maximum signal value, the minimum signal value, or a signal value at a point where the frequency in the histogram is at a maximum. Therefore, a visible radiation image having improved image quality, particularly a quality allowing for a high diagnostic efficiency and accuracy, can be reproduced by adjusting, on the basis of the histogram, the conditions under which the final read out is carried out such as the read-out gain and the scale factor and/or the conditions under which the image signal is processed.
On the other hand, in the course of radiation image recording, it is often desirable to prevent portions of an object not related to a diagnosis or the like from being exposed to radiation. Further, when portions of the object not related to a diagnosis or the like are exposed to radiation, the radiation is scattered by such portions to the portion related to the diagnosis or the like, and the contrast and resolution of the reproduced image are adversely affected by the scattered radiation. Therefore, in many cases, when a radiation image is recorded, the irradiation field is limited to an area smaller than the overall recording region on the stimulable phosphor sheet.
However, in cases where the image input information is ascertained in the manner described above, the problem described below arises. When an irradiation field is limited to an area smaller than the image recording region on the stimulable phosphor sheet and the preliminary read out is carried out over an area markedly larger than the irradiation field, for example, over the overall image recording region on the stimulable phosphor sheet, the image input information is ascertained incorrectly. Specifically, in the aforesaid case, since the preliminary read out image signals at regions outside of the irradiation field are also included in the histogram, the histogram does not accurately represent the actual image input information stored within the irradiation field.
The applicant has proposed various methods for recognizing an irradiation field such as the method disclosed in, for example, Japanese Unexamined Patent Publication No. 61(1986)-39039. The aforesaid problem can be eliminated by automatically recognizing the irradiation field by use of the proposed methods, and carrying out the preliminary read out only over the region thus recognized.
On the other hand, in the course of radiation image recording on a recording medium such as stimulable phosphor sheets, subdivision image recording is often carried out. In subdivision image recording, the recording area on the stimulable phosphor sheet is divided into a plurality of predetermined subdivisions, and the respective subdivisions are exposed to radiation in order to record images thereon. Subdivision image recording is economical since, for example, when an image of a small portion of an object is recorded on a large stimulable phosphor sheet, images of a plurality of object portions may be recorded on a single stimulable phosphor sheet. Also, the radiation image recording and read-out processing speed becomes high.
However, when irradiation fields are limited during subdivision image recording, the respective irradiation fields become separated from each other. FIG. 5 shows the condition of a single stimulable phosphor sheet 103 in a case where subdivision image recording was carried out by dividing the recording region on the stimulable phosphor sheet 103 into two subdivisions, and the irradiation field was limited in each subdivision image recording step. In FIG. 5, B1 and B2 denote the respective irradiation fields. In most conventional methods for recognizing an irradiation field, the recognition process is carried out on the assumption that a single irradiation field is present on a single stimulable phosphor sheet. With such methods, irradiation fields such as those shown in FIG. 5 are recognized incorrectly. On the other hand, a method for automatically recognizing a plurality of irradiation fields on a single stimulable phosphor sheet has also been proposed. However, with the proposed method, the algorithm for recognition of the irradiation fields becomes very complicated, and a very expensive apparatus is necessary for executing the method.
In cases where information on the positions of the respective subdivisions is entered manually into an irradiation field recognizing apparatus, processing for detecting a single irradiation field in each subdivision may be carried out, and the problem of the algorithm's for recognition of the irradiation fields becoming very complicated can be eliminated. However, it is very troublesome to manually enter the subdivision pattern each time radiation image read out from the stimulable phosphor sheet is to be carried out.
The problems described above arise also when radiation images are read out from recording media different from stimulable phosphor sheets.